The present invention relates to a light measuring circuit adapted to be used in an automatic exposure time control device for a photographic camera, and more particularly pertains to the light measuring circuit which means scene light passing through an objective and reflected at the film surface and/or the surface of the shutter curtain or blade located at the exposure aperture of the camera.
In general, a film surface located at an exposure aperture of a focal plane shutter camera is covered with a shutter curtain or blade before exposure is initiated, and after the shutter is released to open, the film surface is uncovered gradually, with the area of film surface to be exposed to scene light increasing gradually to the full area. Thus, if one desires to measure scene light through an image of the object scene projected on the film surface at the exposure aperture, he must take account of the reflecting power of the shutter curtain or blade, the because exposure time must be counted from the moment of shutter initiation when the film surface is fully covered.
There is proposed in U.S. Pat. No. 3,687,026 a camera exposure time control device which has the light measuring system as mentioned above and in which the surface of the shutter curtain has a reflecting power between 23-31%, and preferably 27%. According to the U.S. Patent, the reflecting powers most of films available in the market have reflecting powers within a narrow range as mentioned above, and if the reflecting power of the shutter curtain is selected within that range, then exposure error due to the difference of the reflecting power of the surfaces of film and shutter curtain is negligible in substance. However, a noticeable error may occur in such proposed device for the film having a reflecting power near the outer limits of the range. Further, if the reflecting power of films is greatly changed from that of films now on market, the light measuring device or camera designed to be adapted to the present films will not be able to be used any longer. In addition, such limitation with respect to the reflecting power of the shutter curtain will restrict the design flexibility of the camera, because one may desire to select low reflecting power for avoiding flare of light due to reflection of light at the interior of camera body, or another may desire high reflecting power to allow as much light as possible to impinge on a light receiving element.
Moreover, a Japanese laid-open-patent specification No. 50-86342 which, was published on July 11, 1975, discloses an exposure control device having first and second light integrating capacitors connected in series with each other to a light measuring circuit. The first capacitor is started to be charged upon initiation of shutter opening, while the second is started to be charged when the leading shutter curtain has traveled a predetermined distance or amount, so that compensation may be made for the difference of reflecting power of the surfaces of film and the shutter curtain while the shutter is operating. However, this disclosed device has the partial disadvantage that the capacitance of the second capacitor must be changed for each film having a different reflecting power accordingly, the device should be provided with a plurality of capacitors which will increase the volume or space where the device is arranged. However, it is difficult to obtain a variable capacitor suitable for use in such a control device as usually is incorporated in a camera.